ABSTRACT NCI has nearly 50 ongoing grants to develop and test remotely delivered, technology-based smoking cessation interventions (e.g., apps, websites, text messaging). Remote cessation trials face one key methodological limitation that undermines rigor: the need for biochemical verification of smoking status to accurately assess intervention efficacy. Funding agencies are now strongly encouraging, if not requiring, biochemical verification of smoking in all cessation trials regardless of whether the intervention is delivered in-person or remotely. Incorporation of biochemical indicators of smoking status is critically important as inaccurate assessment of smoking status may lead to dissemination of ineffective treatments and stagnant rates of population-level cessation. Remote collection of expired-air carbon monoxide (CO) is a non-invasive approach that can be used to verify smoking status. Remote CO offers the potential to inform episodic assessment of smoking (e.g., at 1, 3, or 6 months) as is typically done for clinical trial endpoints. Beyond clinical trials, remote CO offers the potential to enhance human-lab methods by providing ecological granular assessment of day-to-day fluctuations in smoking. However, extant trials that have implemented remote methods to assess CO have utilized CO monitors that would be cost-prohibitive ($700-$1,200) to include in large-scale remote trials. Smartphone-enabled CO monitors have recently become available and could dramatically improve the feasibility of remote CO collection. Such monitors are available at substantially lower cost (~$72) than traditional monitors, can be used with any iOS- or Android-compatible mobile device, and detect continuous CO concentrations of 0-100 parts per million. Although these new monitors expand the methodologic potential to capture CO remotely, several issues must first be resolved. Most critically, CO collection via smartphone-enabled monitor must be: 1) integrated in real- time with other research outcomes, 2) valid when compared to gold-standard approaches for biochemical verification, and 3) feasible as applied both to granular and episodic data collection. We herein propose to develop and refine an integrated system through which a smartphone-enabled CO monitor (iCO? Smokerlyzer) is paired with secure online data capture via REDCap. This system will 1) initiate an iCO? reading, 2) video record the participant providing a CO reading, 3) translate the iCO??s raw signal into a CO level, and 4) save the CO level to a REDCap database integrated with other assessments. Subsequently, we will examine: a) validity of the remote CO data capture system as compared to gold-standard methods for biochemical verification and b) compliance with remote CO monitoring as applied both to episodic and granular data collection. This project will develop and validate a tool that could fill the gap for a low-cost, feasible method to biochemically verify smoking status within the context of NIH?s growing portfolio of remote and/or technology-enhanced cessation treatments and has clear potential to improve the rigor of remote assessment of smoking behavior.